Chapter 7
Characterization of Aquatic Humic and Fulvic Materials by Cylindrical Internal Reflectance Infrared Spectroscopy
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Nancy A. Marley, Jeffrey S. Gaffney, and Kent A. Orlandini Environmental Research Division, Argonne National Laboratory, Building 203, 9700 Cass Avenue, Argonne, IL 60439 Cylindrical internal reflectance (CIR) techniques have been applied to humic and fulvic acids that were size fractionated by using hollow-fiber ultrafiltration methods with cutoffs of 0.1 μm and 100,000, 30,000, 10,000, 3,000, and 500 molecular weight. The dissolved organic carbon and major cation contents were compared with the CIR spectra to estimate the active carboxylate units in each size fraction. Comparison of infrared spectra at various pH values for aquatic humics and for model polycarboxylate compounds (polymaleic acid and polyacrylic acid) indicated that the principal metal binding functionalities are carboxylate groups. Humic and fulvic materials naturally present in ground and surface waters can act as strong complexing agents for metals and radionuclides and therefore can increase their migration and transport of these species in the geosphere (7-7). The distribution of bound metals and radionuclides in natural waters varies across the size distribution of humic materials. In most cases, the small organics (3,000 molecular weight) are the most active complexing agents (7-10). Evidence suggests that the carboxylate functional groups, which are mainly responsible for the aqueous solubility of these natural organics, are the most active complexing sites within the humic and fulvic acid molecules, with the smaller fulvic acids possessing the highest percentage of carboxylates (11-13). Vibrational spectroscopy is the method of choice for the characterizing functional groups in complex organic molecules. Infrared transmission spectroscopy has been used on dried humics pressed into K B r pellets to determine the relative carboxylate content of humic materials (14-16). However, interferences arise from the presence of water bands and possible alterations of the samples under the high pressures used to form the pellets. Diffuse-reflectance techniques can avoid some of the difficulties associated with the KBr pressed-pellet method (9,17-18). To obtain a spectrum analogous to an absorption spectrum, the data are transformed from reflectance units to Kebulka-Munk (K-M) units. However, K - M units are related to 0097-6156/96/0651-0096$15.00/0 © 1996 American Chemical Society
In Humic and Fulvic Acids; Gaffney, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
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7.
MARLEY ET AL.
Characterization of Aquatic Materials by CIR Techniques
the molar absorption coefficient and the scattering coefficient, which vary with particle size and packing density of the sample. There is concern that the K - M transformation of data tends to amplify strong absorption bands (small relative reflectance) over weak ones (large relative reflectance) and may therefore bias any quantitative interpretation of results (77). Raman spectroscopy has been used to characterize large organic molecules analogous to humic and fulvic acids in aqueous solution (19). However, humic materials absorb at the visible wavelengths used for laser Raman, and the resulting fluorescent background overwhelms any Raman signals. Fourier transform Raman, with near-infrared lasers, has been used successfully for other highly fluorescent macromolecules (20), but a recent attempt to characterize humic and fulvic acids by this technique yielded uncertain results (21). Although the absorption of liquid water yields too strong a background signal to permit infrared spectroscopy of aqueous solutions by traditional techniques, the recent development of cylindrical internal reflectance (CIR) overcomes these limitations. This method permits the quantitative study of aqueous solutions by probing the interface between the water solution and an internal reflectance crystal, effectively providing a highly reproducible cell with an extremely short optical path length (22). The CIR techniques have been used to study the behavior of Aldrich humic acid in aqueous solution (23) and to quantitatively determine the carboxylate content of humic and fulvic materials obtained from surface waters (7,24). For this study, humic and fulvic materials obtained from a small glaciated bog were separated into five size fractions by using hollow-fiber ultrafiltration techniques. The major cations associated with these organics are reported as a measure of the natural binding capacity of each size range. The structural characteristics and carboxylate content of each group were studied by CIR spectroscopy; the results are compared with those obtained by traditional Fourier transform infrared techniques. To aid in spectral interpretation, results were compared to those for selected model polyelectrolytes and simple acids. Sampling and Isolation of Aquatic Humic and Fulvic Materials Water samples were obtained from Volo Bog, located in an Illinois nature preserve northwest of Chicago. The small glaciated bog is surrounded by sedge peat and has no surface inlet or outlet. The water is of low nutrient content and has a pH of 4-5. Water samples were first passed through a 35 μηι screen to remove large particulates and microorganisms, then prefiltered with a 0.45 μιη Millipore filter to remove suspended solids. Humic materials were concentrated from 60 gallon water samples with hollowfiber filter cartridges (Amicon Division, W.R. Grace and Co.) with effective size cutoff diameters of 0.1 μηι and 100,000, 30,000, 10,000, and 3,000 nominal molecular weight. A n additional flat-disk filter membrane in a vortex mixing stirred cell was used to separate the 500 nominal molecular weight species.
In Humic and Fulvic Acids; Gaffney, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
98
HUMIC AND FULVIC ACIDS
Experimental Methods Dissolved organic carbon (DOC) measurements were obtained on the size-fraction ated concentrates with a PHOTOchem Organic Carbon Analyzer (Sybron, Model E3500) as an indication of the concentration of humic materials in each fraction. DOC values are reported as ppmC (mgL" ) The major cations in each fraction were determined by inductively coupled plasma spectroscopy (Instruments SA, Model JY 86). Detailed experimental procedures for obtaining infrared spectra on humic and fulvic acids have been reported previously (9,22,25-26) and will be briefly described here. Infrared spectra were taken on the size-fractionated samples by using a Fourier transform infrared spectrometer (Mattson, Polaris) with a cooled Hg/Cd/Te detector. Dried humic and fulvic materials were studied by diffuse reflectance infrared spectroscopy (Spectra Tech DRIFT accessory) and reported in K - M units, as well as by transmission absorbance in a KBr pellet. Infrared absorption spectra were obtained directly on the aqueous size-fractioned concentrates with CIR (Spectra Tech CIRCLE accessory). Raman spectra were taken by using an argon ion laser (Spectra-Physics Model 2025-05), a triple-grating monochromator (Spex Triplemate Model 1877), and a photodiode array detector system (Princeton Applied Research Model 1420). A l l Raman and infrared spectra were taken at 2 cm" resolution.
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Characterization of Dissolved Organic Carbon and Trace Elements The total concentration of dissolved organic carbon (DOC) found in the aqueous fractions from Volo Bog (0.45 μιη) was 25 ppm. The DOC was distributed among the five size fractions as shown in Table I. Table I Dissolved Organic Carbon (DOC) and Major Elements in Sizefractionated Samples from Volo Bog. Total μπι) ppm
(
